Energy System Basics

Why do we do workouts of different lengths? Why is it sometimes long, sometimes broken into intervals? Because we have four (4) distinct ways we produce energy, and need to develop all of them to be the best we can be.

The human body fuels physical activity primarily by turning a molecule called adenosine triphosphate (ATP) into adenosine diphosphate (ADP). This releases energy which our bodies are able to use to create motion, digest food, think, and do all the other things we do. Exactly how that happens, we don’t know. But I’m typing this and you’re reading this, so it’s working for both of us. There are two major sources of fuel the body uses to create ATP: sugar and fat. There are also two ways to turn sugar into ATP: aerobically and anaerobically. Thus, we have four sources of ATP, plus the actual turning of ATP into ADP, so we can think about five (5) different processes acting inside us to turn food into energy. All of these processes require numerous enzymes and other chemicals and structures in our bodies to function. We vary the duration and intensity of our workouts to tell our bodies to get good at producing energy in every way it can.

All five of these systems are always active. What changes is the ratio between them – which system is producing what percentage of our energy. As we go from rest to hard work, the balance between energy sources changes. Textbooks often describe these systems from the “fastest” to the “slowest,” which may be confusing. We are better off starting with our default energy system, aerobic lipolysis, which is the fancy way to say ‘fat burning with oxygen,’ and the other systems increase their contributions to energy production as effort increases.

Aerobic lipolysis burns fat for energy, and can theoretically power the body until the mind collapses, and keep you alive until you come to. It just does it really slowly, meaning that using this system may not feel like exercising. This is the “fat burning” setting on exercise equipment. If you are asleep, sitting, moving easily, or can speak in full sentences, your energy should be coming from aerobic lipolysis (more on that ‘should’, in a subsequent post).

This is a rough depiction of the balance between energy systems.

As intensity of effort increases, the body needs more energy more quickly. Aerobic lipolysis continues, but the body increases the amount of sugar it burns with oxygen, a process called aerobic glycolysis. This is faster than burning fat, but has a very limited supply of energy. Even highly trained athletes can only store about two (2) hours worth of sugar. In the graph above, aerobic lipolysis and glycolysis are lumped together as “oxidative” because oxygen is used in these “aerobic” (“with oxygen”) processes.

If intensity continues to increase, we begin to burn sugar without using oxygen in the chemical reaction, called anaerobic glycolysis. This produces pyruvate and lactate, both of which can also be used for fuel. This system produces large amounts of energy quickly, but its side products build up in the muscles, causing a burning sensation which eventually overwhelms the mind’s ability to continue the effort. There are two distinct chemical pathways involved, so you may hear of “fast glycolysis” and “slow glycolysis” or similar terms. Whichever pathway is predominating, anaerobic glycolysis can only power the body for about one to two minutes, depending on fitness level.

Finally, the phosphagenic process (aka ATP-CP cycle, aka phosphocreatine system), is where energy is actually put to work. ATP is split to produce energy here. A molecule called “creatine phosphate” is available in small quantities to replenish the phosphagen and turn ADP back into ATP as quickly as possible. If you’ve heard of creatine supplementation, it is intended to make more creatine phosphate available and thus help this energy system provide more energy for longer. It can still only last about 10 seconds at maximum effort.

What does all this mean for your workouts? To be the best athlete you can be, you need to develop all of your energy systems, at least to a minimum level. The weightlifter who gets winded going up a flight of stairs can’t recover and train as hard as he could if he did some aerobic work to hasten recovery between sets. The endurance athlete who doesn’t challenge the phosphagenic or glycolytic energy systems will have only one speed: slow. While able to go for a very long time, that athlete will also lose any sprints, struggle on steep hills, have poor reaction times, and generally be less capable than otherwise.

How do we do this? Watch for the next blog, or contact me with questions!